INTRODUCTION
Radioactive nuclides, or radionuclides, are species of unstable atomic nuclei without the restriction of being forms of the same element. Radioactive nuclides consist of all the sets of radioactive isotopes. Radiochemistry a subdivision of chemistry which deals with the study of radioactive substances. it includes the nuclear transformation involved, transmutation of one element into another,and the nature and properties of the radiation emitted. it also deals with the use of this radiation in chemical tracer analysis, for geological & archeological (chemical dating), and for initiation of cross-lining(polymerazation) [1] Spontaneous nuclear transformation of nuclide into another nuclide, accompanied by emission of nuclear radiation ,either corpuscular or electromagnetic. it may be natural, as with radium, artificial (caused bybombardment of a stable nucleus with neutrons or deuterons), or induced, as in radioactive carbon.the emanations are in the form of alpha, beta, gamma rays. the natural radioactive elements are uranium, radium, radon and thorium(the principal members of the uranium decay series), the ultimate end products being stable isotope of elements,e.g.,sodium,iodine etc., can be made radioactive by bombardment with neutrons, deuterons.or other heavy particles. Radionuclides, mainly 3H & 14C, are widely used as tracers in analysis, and in distribution and metabolism studies of drugs in animals.such isotope with long half-lives are not suitable for use in human medicine,but a number of radioisotopes with comparatively short half-lives, measured in hours or days,are now widely used in radiopharmaceutical preparations as diagnostic agents and in the treatment of neoplastic disease. these include 32P,51Cr,57Co,59Fe,75Se,125I.131I & 99mTc. [2] Isotopes are generally distinguished by three analytical means. The first of them makes use of radioactive isotopes, such as tritium (3H), 14C, 32P etc. This is a highly sensitive technique, but special facilities are required to handle radioactive material. Mass spectroscopy can also be used to detect isotopes. This is also a highly sensitive technique. When the fragmentation pattern of a compound is known, mass spectral data provide a wealth of information. The third, and at present the most frequently used technique is nuclear magnetic resonance. This technique requires an NMR active nucleus such as 2H, 13C, 17O etc. and is relatively less sensitive. But the ease of operation more than compensates for its limitations.[3]
According to the different condition for storage ,handling,disposal and the manner in which the radionuclide are used they are categorized into two groups (1)sealed (2)unsealed. In radio therapy sealed radio isotope are used that are encapsulated to prevent the loss of the radioisotopes. On the other hand most radiopharmaceuticals are used in unsealed state. i.e. the radio isotope is present in the liquid , particular or gaseous state. These offers some hazards which include contamination by skin contact and accidental inhalation or ingestion.[4]
Types of radioactive isotopes by origin
1) Long-lived radioactive nuclides
Some radioactive nuclides that have very long half lives were created during the formation of the solar system (~4.6 billion years ago) and are still present in the earth. These include 40K (t½ = 1.28 billion years), 87Rb (t½ = 48.8 billion years), 238U (t½ = 447 billion years), and 186Os (t½ = 2 x 106 billion years, or 2 million billion years).
2) Cosmogenic
Cosmogenic isotopes are a result of cosmic ray activity in the atmosphere. Cosmic rays are atomic particles that are ejected from stars at a rate of speed sufficient to shatter other atoms when they collide. This process of transformation is called spallation. Some of the resulting fragments produced are unstable atoms having a different atomic structure (and atomic number), and so are isotopes of another element. The resulting atoms are considered to have cosmogenic radioactivity. Cosmogenic isotopes are also produced at the surface of the earth by direct cosmic ray irradiation of atoms in solid geologic materials.
Examples of cosmogenic nuclides include 14C, 36Cl, 3H, 32Si, and 10Be. Cosmogenic nuclides, since they are produced in the atmosphere or on the surface of the earth and have relatively short half-lives (10 to 30,000 years), are often used for age dating of waters.
3) Anthropogenic
Anthropogenic isotopes result from human activities, such as the processing of nuclear fuels, reactor accidents, and nuclear weapons testing. Such testing in the 1950s and 1960s greatly increased the amounts of tritium (3H) and 14C in the atmosphere; tracking these isotopes in the deep ocean, for instance, allows oceanographers to study ocean flow, currents, and rates of sedimentation. Likewise, in hydrology it allows for the tracking of recent groundwater recharge and flow rates in the vadose zone. Examples of hydrologically useful anthropogenic isotopes include many of the cosmogenic isotopes mentioned above: 3H, 14C, 36Cl, and 85Kr.
4) Radiogenic
Radiogenic isotopes are typically stable daughter isotopes produced from radioactive decay. In the geosciences, radiogenic isotopes help to determine the nature and timing of geological events and processes. Isotopic systems useful in this research are primarily K-Ar, Rb-Sr, Re-Os, Sm-Nd, U-Th-Pb, and the noble gases (4H, 3H-3He, 40Ar).
Because of their stable evolution in groundwater, such naturally occurring isotopes are useful hydrologic tracers, allowing evaluation of large geographic areas to determine flowpaths and flow rates. Consequently, they are helpful in building models that predict fracturing, aquifer thickness, and other subterranean features.
Production of radioisotope
Production of radioisotopes includes three principle categories, which are (1) neutron activation (bombardment), (2) fission product separation, and (3) charged particle bombardment. Nuclear bombardment constitutes the major method for obtaining industrially important radioisotope materials. Radioisotopes may exist in any form of matter, with solid materials comprising the largest group.
Emission of radioisotope
Three main type of radiation from radioactive substance are alpha(α),beta(β) and gamma(γ) rays.most source emit more than one type of emission, The penetrating power of each radiation varies considerably according to it’s nature and it’s energy. Alpha particle are completely absorbed in a thickness of a few micrometers to some tens of micrometer of solid or liquid. Beta particle are completely absorbed in a thickness of several millimeter to several centimeter. Gamma rays are not completely absorbed but only attenuated. The denser the absorbent , the shorter the range of alpha, beta particle and greater the attenuation of gamma rays.[5]
Cause of desirable effect of shorter half-lives radionuclide
In general, radionuclides (RN) with shorter half-lives are desirable for use in diagnostic nuclear medicine (NM) because they usually produce less total dose to the patient, thus yielding reduced biological impact compared to longer-lived radionuclides. Radionuclides that are selected for diagnostic NM procedures preferably emit photonic radiation (usually gamma rays) that have an energy in the approximate range from 100 to 150 keV. This energy is desirable since it is high enough to ensure reasonable penetration of the human body so that photons are able to reach the imaging device, usually a gamma camera; additionally this energy is detected with high efficiency by the detectors in the cameras and is low enough so that camera collimators work effectively to record primarily photons moving in a direction more-or-less perpendicular to the face of the detector. This is necessary in order to obtain acceptably sharp images without appreciable blurring. A final desirable property is that the radionuclide emits minimal amounts of particulate radiation such as beta particles and alpha particles so as not to produce excess patient dose while doing nothing to improve image quality. There are both long- and short-lived radionuclides that could fulfill these recommendations.
